ABSTRACT Troubling child abuse statistics underscore that parent-child aggression (PCA) remains pervasive in the US. To protect children, efforts to prevent child abuse must tackle the most critical factors that lead parents to engage in PCA. To accomplish this, a number of research gaps need to be addressed. Most research inquiry has concentrated on maternal risk, overlooking factors relevant to fathers. And methodological limitations have compromised current research conclusions about the true role of any given factor in relation to PCA risk. The proposed project targets this last research gap, designed to provide preliminary evidence for an objective methodological assessment of parental reactivity?a process embedded in this study within a comprehensive model of PCA risk applied to both mothers and fathers. To determine whether a particular factor contributes to PCA risk, we must be confident in whether it was adequately measured. Consequently, the current study will evaluate whether physiological functioning in parents, as measured via salivary biomarkers, can serve as indicators of PCA risk. To date, studies suggest parental physiological reactivity to infant crying relates to harsh parenting, and physiological synchrony between parent and child relates to sensitive parenting. Higher testosterone levels have also been associated with reduced parental involvement. Thus, saliva samples will be assayed to determine levels of testosterone and levels of physiological synchrony between parents and their toddlers on salivary alpha-amylase and cortisol. Participants will be drawn from families enrolled in the Following First Families (Triple-F) study, a prospective longitudinal study on the evolution of PCA risk which has followed families since the last trimester of the mother's pregnancy. A diverse sample of parents was targeted, with half of the families constituting a risk group with sociodemographic disadvantages, to determine whether factors identified during pregnancy are more readily triggered to become abusive in certain groups. Triple-F participants were re-assessed when the child was 6 months and 18 months to investigate risk and resilience factors in new mothers and fathers that may shift during the transition to parenthood. Capitalizing on the rich dataset already available from these time points, salivary biomarkers from the parents will be linked to their self-report questionnaires, implicit and behavioral analog tasks, and coded observations of parent-child interactions. From the Triple F study sample, saliva will be assayed for salivary alpha-amylase and cortisol collected from 120 mothers and 100 fathers and their 18 month old toddlers, gathered before and after their structured parent-child interaction; saliva will also be assayed for testosterone in parents based on their initial saliva samples. These salivary biomarkers can provide an objective indicator of reactivity that represents an important early process in the theoretical model. Thus, this project will clarify the potential value of this methodological innovation, simultaneously evaluating which factors are most relevant to PCA risk within a theoretically based model, for both mothers and fathers.